Oxford, UK, 4 April 2012, Summit (AIM: SUMM), a UK drug discovery company, today announces a
share issue to raise £5.0 million (before expenses) through the issue of 166,666,670 new Ordinary
Shares by way of a Placing at 3.0 pence per Ordinary Share to certain institutional investors and
Directors. The Placing is being fully underwritten by Singer Capital Markets Limited on, and subject
to, the terms of the Placing Agreement. Hybridan LLP acted as Joint Brokers on this transaction.

The Circular (the “Circular”) relating to the Placing will be posted to shareholders today. The Circular
contains a notice of General Meeting to approve, inter alia, the Placing which will be held at the
offices of Fasken Martineau LLP, 17 Hanover Square, London, W1S 1HU on 20 April 2012 at 10:00
a.m.

The Circular will soon be available to view on the Company’s website (www.summitplc.com).Copies
will also be available from the offices of Summit, 91 Milton Park, Abingdon, Oxfordshire, OX14 4RY.

Commenting on the proposed Placing, Barry Price PhD, Executive Chairman of Summit said:
“The good progress made in the development of our key drug programmes and Seglin™ technology
means we have a number of assets that are expected to reach key development milestones over the
coming months. This proposed fundraise, combined with the strengthening of the executive
management team following the appointment Glyn Edwards as Chief Executive, will help to ensure
the Company is able to capitalise on the potential of these assets and generate value for all our
shareholders.”

A webcast presentation for investors will be available from the Company’s website from 10:00 a.m.
BST today.

For more information, please contact:

Summit

Barry Price, PhD,

Glyn Edwards

Richard Pye, PhD

Tel: +44 (0)1235 443 939

Singer Capital Markets

(Nominated Adviser and Joint broker)

Shaun Dobson / Claes Spång

Tel: +44 (0)203 205 7500

Hybridan LLP

(Joint broker)

Claire Louise Noyce / William Lynne

Tel: +44 (0)207 947 4350

Peckwater PR

(Financial public relations)

Tarquin Edwards

Tel: +44 (0)7879 458 364

tarquin.edwards@peckwaterpr.co.uk

About Summit

Summit is an Oxford, UK based drug discovery Company with an innovative Seglin™ technology
platform for the discovery of new medicines and a portfolio of drug programme assets. Summit’s
programme portfolio consists of a number of drug programmes targeting high-value areas of unmet
medical need including Duchenne Muscular Dystrophy and C. difficile infection. Summit is listed on
the AIM market of the London Stock Exchange and trades under the ticker symbol SUMM. Further
information is available at www.summitplc.com.

Forward Looking Statements

This contains forward-looking statements. These statements relate to, among other things, analysis
and other information that are based on forecasts of future results and estimates of amounts not yet
determinable. These statements also relate to the Company’s future prospects, developments and
business strategies. Forward-looking statements are identified by their use of terms and phrases
such as “anticipate”, “expect”, “believe”, “could”, “envisage”, “estimate”, “intend”, “may”, “plan”, “will”
or the negative of those, variations or comparable expressions, including references to assumptions.

The forward-looking statements in this document are based on current expectations and are subject
to risks and uncertainties that could cause actual results to differ materially from those expressed or
implied by those statements. These forward-looking statements speak only as at the date of this
document. The Company cannot guarantee that its forward looking statements will not differ
materially from actual results. The Company does not undertake any obligation to update forward-
looking statements or risk factors other than as required by any relevant regulations, whether as a
result of new information, future events or otherwise. The Company cautions you therefore that you
should not rely on any of these forward looking statements as statements of historical fact or as
guarantees or assurances of future performance. Important factors that could cause actual results to
differ materially from those in the forward-looking statements are regional, national, global, political,
economic, business, competitive, market and regulatory conditions

1. Background to and reasons for the Placing

The Group has cash resources which, based on the current levels of cash expenditure, are expected
to last into Q3 2012. As a consequence, the Group requires additional finance to enable its strategy
for creating shareholder value to be implemented in an optimal manner.

It is the intention of the Directors that the amount of £4.6 million (net of costs) being raised from the
Placing, will be used to fund a Phase I study for the Company’s C. difficile programme and to support
the development of the programme targeting the enzyme OGA, for the treatment of Alzheimer’s
disease and related dementias, in addition to providing the Company with sufficient funds to develop
new infectious and rare disease programmes from its Seglin™ technology platform. In addition, an
amount will be set aside for working capital and general corporate and commercial purposes.

The proceeds of the Placing will significantly enhance the Group’s financial position and provide it
with sufficient cash resources to fund the business into Q3 2013.

If the Resolutions are not passed by Shareholders at the General Meeting, the Placing would
be unable to proceed. In this situation, the Company would not have cash resources to
maintain current operations beyond the third quarter of 2012 and would need to consider
alternative strategic options that the Directors believe would not be in the best interests of
stakeholders. These options could include raising finance from alternative sources, disposals
of intellectual property assets, reducing costs through redundancies or other means or a sale
of the Company at a price which the Directors believe would not recognise the potential long-
term value of the business. Any one, or all, of these remedial actions could have a significant
adverse or dilutive effect on the interests of Shareholders and in the valuation of the Company. In addition, should the Company or its subsidiaries be forced to make an
arrangement with creditors or suffer some other insolvent event then title in the most
advanced programmes, C. difficile and/or Duchenne Muscular Dystrophy, could be lost.

Future activities

Placing proceeds of £4.6m (net of costs) will enable the Company to benefit from the results of the
Phase I clinical study of SMT C1100 for the treatment of Duchenne Muscular Dystrophy programme.
Your Board believes that the completion of this Phase I trial represents a key development milestone.
If the Phase I is successful then the Company would seek to commercialise the interest in the
programme through collaboration with other companies to progress SMT C1100 through further
clinical trials in return for an up-front payment, development and regulatory milestones and sales
royalties. Headline data from this study are anticipated in Q3 2012. The Company intends to develop
SMT 19969 through a Phase I study as a treatment for C. difficile infection (‘CDI’). At the end of this
study, which is anticipated to be in H1 2013, the Company may seek partners to further develop the
product in return for an up-front payment, development and regulatory milestones and sales royalties.
The Company further intends to investigate the activity of compounds derived from its Seglin™
technology platform against a number of promising targets. This includes advancing our programme
targeting the enzyme OGA, which is implicated in the progression of Alzheimer’s disease and related
neurodegenerative diseases, through to in vivo proof concept. Reaching this stage is expected to
increase the potential of being able to secure a licensing partner to continue the development of the
programme. In addition, new programmes will be developed from the Seglin platform in targeting
infectious and rare diseases to build on our established expertise in these areas

2. Terms of the Placing

The Placing has been underwritten by Singer and is conditional upon the Resolutions having passed.
The New Ordinary Shares will, when issued, rank pari passu, in all respects with the Existing Ordinary
Shares. Admission is expected to take place and dealing in the New Ordinary Shares on AIM are
expected to commence at 8.00 a.m. on 24 April 2012.

3. Current Trading and Prospects

The Company has made significant progress in the development of the SMT C1100 (DMD
programme) and SMT 19969 (C. difficile infection programme) assets. Both of these assets are set to
enter Phase I clinical studies during the coming period. If these studies are successful then the value
of these assets would increase. In addition, compounds have been identified from our Seglin
discovery programme that show activity against the enzyme OGA that is implicated in the progression
of Alzheimer’s disease and related dementias. The advances made in this programme serve to
highlight the broader potential of Seglin™ technology platform as we work towards establishing new
programmes in other therapy areas.

A notice convening the General Meeting to be held at the offices of Fasken Martineau LLP, 17
Hanover Square, London, W1S 1HU at 10.00am on 20 April 2012 is set out in the Circular.

The Resolutions proposed at the General Meeting are as follows:

1. an ordinary resolution to authorise the Directors, pursuant to section 551 of the Act, to issue
the Placing Shares and the Warrants;

2. a special resolution, pursuant to section 571 of the Act, to disapply the statutory pre-emption
rights on the allotment of equity securities, pursuant to the authority contained in Resolution
1;

3. an ordinary resolution to authorise the Directors, pursuant to section 551 of the Act, to issue
35,408,845 Ordinary Shares in addition to the New Ordinary Shares; and

4. a special resolution, pursuant to section 570 of the Act, to disapply the statutory pre-emption
rights on the allotment of equity securities, pursuant to the authority contained in Resolution 3;

5. Recommendation

The Directors, taking into account the options available to the Company, believe the Placing to be fair
and reasonable so far as the Shareholders are concerned. Lansdowne Partners, a substantial
shareholder in the Company, is subscribing for 44,203,944 Placing Shares in the Placing for a
consideration of £1.3 million. As such, the Placing constitutes a related party transaction under Rule
13 of the AIM rules. The Directors consider, having consulted with Singer, that the terms of the
Placing are fair and reasonable insofar as Shareholders are concerned.

Accordingly, the Directors unanimously recommend Shareholders to vote in favour of the
Resolutions as the Directors intend to do in respect of their beneficial shareholdings amount
to 10,020,951 Ordinary Shares, representing approximately 5.3 per cent of the Company’s
existing issued ordinary share capital.

EXPECTED TIMETABLE OF PRINCIPAL EVENTS

2012

Announcement of the Placing and Posting 4 April

General Meeting 10.00 a.m. on 20 April

Admission and commencement of dealings of the Placing Shares 23 April

Notes:

(1) References to times in this Announcement are to London time (unless otherwise stated).

(2) If any of the above times or dates should change, the revised times and/or dates will be notified
by an announcement to an RIS.

(3) The timing of the events in the above timetable and in the rest of this Announcement is indicative
only.